FERGUSON, Mo. >> Attorneys for Michael Brown’s family vowed Tuesday to push for federal charges against the police officer who killed the unarmed 18-year-old, while the officer insisted in his first public statements that he could not have done anything differently in the confrontation.

As darkness fell, hundreds more National Guardsmen were ordered into Ferguson in hopes that their presence would help local law enforcement keep order in the St. Louis suburb.

During an interview with ABC News, Wilson said he has a clean conscience because “I know I did my job right.”

Wilson, who has been on leave since the Aug. 9 shooting, had been with the Ferguson police force for less than three years. He told ABC that Brown’s shooting marked the first time he had fired his gun.

The Brown family attorneys said the grand jury process was rigged from the start to clear the white officer, Darren Wilson, in the Aug. 9 shooting death of Brown, who was black. They criticized everything from the evidence the St. Louis County prosecutor presented to the jury to the way it was presented, as well as the timing of the announcement of the grand jury’s final decision.

“We said from the very beginning that the decision of this grand jury was going to be the direct reflection of the presentation of the evidence by the prosecutor’s office,” attorney Anthony Gray said. He suggested the county’s top prosecutor, Bob McCulloch, presented some testimony to discredit the process, including from witnesses who did not see the shooting.

They hope a federal civil rights investigation results in charges against Wilson.

Twelve commercial buildings in Ferguson burned down during protests that erupted after the grand jury’s decision was announced, and firefighters responded to blazes at eight others, fire officials said. Other businesses were looted, and 12 vehicles were torched.

By Tuesday afternoon, several businesses along West Florissant Avenue, scene of many of the most intense clashes, were already closed.

Natalie DuBose, owner of Natalie’s Cakes and More, planned to spend Tuesday night at her business, which is only about a block from the Ferguson Police Department.

“I have to be here because I have orders that I have to complete for tomorrow and for Thursday because of the holiday. I just couldn’t do it today because of the cleanup.”

A window at her business was busted out in Monday night’s turmoil.

“I’m emotional because of what has happened to me,” she said. “This is my livelihood. This is the only source of income I have to raise my children.”

In the aftermath of Monday night, Missouri Gov. Jay Nixon more than tripled the number of National Guard troops in Ferguson, ordering the initial force of 700 to be increased to 2,200.

“Lives and property must be protected,” Nixon said. “This community deserves to have peace.”

Authorities made 61 arrests in Ferguson overnight, many for burglary and trespassing, and 21 in St. Louis, where protesters broke some store windows along South Grand Avenue.

At least 18 people were injured and sought treatment at area hospitals, including one person who was shot.

Brown’s parents made public calls for peace in the run-up to Monday’s announcement, and on Tuesday, their representatives again stressed that those setting fires and engaging in violence were not on Michael Brown’s side.

Protests continued during the day on Tuesday. In Clayton, where the grand jury met, clergy members and others blocked morning traffic for several hours. In downtown St. Louis, where demonstrators swarmed the steps of a federal courthouse and stopped traffic, at least four people were arrested.

Many area districts canceled classes out of concern for the safety of students traveling to and from school.

The grand jury’s decision means Wilson will not face any state criminal charges for killing Brown, whose death inflamed deep racial tensions between many black Americans and police.

Attorneys for Brown’s family said they hope an ongoing federal civil rights investigation leads to charges. But federal investigations of police misconduct face a steep legal standard, requiring proof that an officer willfully violated a victim’s civil rights.

Testimony from Wilson that he felt threatened, and physical evidence such as the discovery of Brown’s blood in Wilson’s patrol car, almost certainly complicates any efforts to seek federal charges.

Under federal law, “you have to prove as a prosecutor that the officer knew at the moment that he pulled the trigger that he was using too much force, that he was violating the Constitution,” said Seth Rosenthal, a former Justice Department civil rights prosecutor.

The Justice Department has also launched a broad probe into the Ferguson Police Department, looking for patterns of discrimination.

Attorney General Eric Holder said the department aims to complete those investigations as quickly as possible “to restore trust, to rebuild understanding and to foster cooperation between law enforcement and community members.”

Regardless of the outcome of the federal investigations, Brown’s family also could file a wrongful-death lawsuit against Wilson.

Speaking in Chicago, President Barack Obama said he knows the grand jury’s decision “upset a lot of people” but that the “the frustrations that we’ve seen are not just about a particular incident. They have deep roots in many communities of color who have a sense that our laws are not always being enforced uniformly or fairly.”

Wilson’s lawyers issued a statement praising the decision and saying the officer, who has remained out of the public eye since the shooting but gave an interview that was to air on ABC Tuesday evening, is grateful to his supporters.

“Law enforcement personnel must frequently make split-second and difficult decisions,” the lawyers wrote. “Officer Wilson followed his training and followed the law.”

Scott Holtgrieve, a St. Louis County man who attended an August fundraiser on Wilson’s behalf, always viewed with skepticism witness accounts that Wilson shot Brown while Brown held his hands up in a form of surrender and was on his knees.

“What they were saying just didn’t seem rational — that an officer would shoot someone in cold blood that way at point-blank range, especially in that neighborhood where you know a lot of people are watching,” Holtgrieve said.

The protests that followed the grand jury decision quickly became chaotic, with protesters looting and setting fire to businesses and vehicles, including at least two police cars. Officers eventually lobbed tear gas from inside armored vehicles to disperse crowds.